We have numbers 3623 and 9021. Let's start adding them together.

Step 1: We'll start by adding the digits 3 & 1 in column 1 and get 4.

Step 2: We'll start by adding the digits 2 & 2 in column 2 and get 4.

Step 3: We'll start by adding the digits 6 & 0 in column 3 and get 6.

Step 4: We'll start by adding the digits 3 & 9 in column 4 and get 12.
We'll write down the last digit 2 and carry the 1 to the next column.

So, 3623 + 9021 = 12644.

